Re: bcache strange behaviour in write back mode

[Kent Overstreet <koverstreet-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>]

On Mon, Apr 22, 2013 at 10:27:03PM +0200, Jack Wang wrote:
> Thanks for reply, Kent, two of my colleagues saw this behaviour, so I
> think we can reproduce this.
> If you could give me more detailed guide to narrow it down, I can try it
> on my side.

So, my current hypothesis is that the problem is the allocator spinning,
and the IO is from it continually rewriting prios/gens.

But I'm still not sure what's causing the allocator to spin, that's what
the last patch was supposed to fix.

Can you see if you can reproduce it with this patch, and then tell me
what shows up in the dmesg log? I expect you'll get a _lot_ of output -
flip timestamps on in your kernel config, if they're not already on.
Thanks!

commit 60a09d37d301f88dd0f0f413408821a067966d1a
Author: Kent Overstreet <koverstreet-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Date:   Mon Apr 22 14:49:33 2013 -0700

    bcache: Allocator debug patch

diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
index 2879487..37c22c6 100644
--- a/drivers/md/bcache/alloc.c
+++ b/drivers/md/bcache/alloc.c
@@ -393,12 +393,15 @@ void bch_allocator_thread(struct closure *cl)
 				allocator_wait(ca, !list_empty(&ca->discards));
 				do_discard(ca, bucket);
 			} else {
-				fifo_push(&ca->free, bucket);
+				BUG_ON(!fifo_push(&ca->free, bucket));
 				closure_wake_up(&ca->set->bucket_wait);
 			}
 		}
 
 		allocator_wait(ca, ca->set->gc_mark_valid);
+
+		printk(KERN_DEBUG "bcache: invalidating buckets: free_inc %zu/%zu\n",
+		       fifo_used(&ca->free_inc), ca->free_inc.size);
 		invalidate_buckets(ca);
 
 		allocator_wait(ca, !atomic_read(&ca->set->prio_blocked) ||
@@ -407,8 +410,12 @@ void bch_allocator_thread(struct closure *cl)
 		if (CACHE_SYNC(&ca->set->sb) &&
 		    (!fifo_empty(&ca->free_inc) ||
 		     ca->need_save_prio > 64)) {
+			printk(KERN_DEBUG "bcache: writing prios: free_inc %zu/%zu\n",
+			       fifo_used(&ca->free_inc), ca->free_inc.size);
 			bch_prio_write(ca);
-		}
+		} else
+			printk(KERN_DEBUG "bcache: not writing prios: free_inc %zu/%zu\n",
+			       fifo_used(&ca->free_inc), ca->free_inc.size);
 	}
 }